Microelectronic elements such as packaged or bare semiconductor chips, discrete components and other elements are mounted to components such as rigid or flexible circuit panels by many different processes. For example, in a so-called xe2x80x9cflip chipxe2x80x9d or xe2x80x9cC4xe2x80x9d technique, a bare semiconductive chip is mounted to a circuit panel by disposing solder balls onto contact pads of the chip or circuit panel. The chip is then mounted with its front or contact-bearing surface facing downwardly, toward the first surface of the circuit panel and with the contact pads of the chip aligned with the contact pads of the circuit panel. The assembly is then heated to melt or xe2x80x9creflowxe2x80x9d the solder whereupon the assembly is cooled leaving solid solder masses connecting each contact pad of the chip with the corresponding contact pad on the circuit panel.
As described, for example, in Multi-Chip Module Technologies and Alternatives; the Basics (Doame and Franzon, eds., 1993, pp. 450-476), considerable effort has been devoted in the art to development of the flip chip technique. Nonetheless, the flip chip technique typically requires large solder balls to provide solder joints having the strength and fatigue resistance needed to accommodate differential expansion and contraction of the chip and the circuit panel caused by temperature changes during service and/or manufacture. Thus, the flip chip technique typically requires contact pads having large center to center spacings or xe2x80x9cpitch.xe2x80x9d For these and other reasons, use of the flip chip technique has been limited.
As described, for example, in certain embodiments of commonly assigned U.S. Pat. Nos. 5,148,265, 5,148,266 and 5,915,752, the disclosures of which are hereby incorporated by reference herein, microelectronic components such as semiconductor chips may be connected to components referred to as xe2x80x9cinterposersxe2x80x9d such as flexible dielectric elements having terminals thereon. These connections desirably are made so that the terminals on the connection components remain movable with respect to the chip. A layer of a compliant material as for example, a gel or an elastomer may be provided between the interposer and the microelectronic element. These terminals can then be bonded to contact pads of a larger substrate such as a larger circuit panel. Because the terminals of the interposer remain movable with respect to the chip differential expansion in use does not impose substantial stresses on the connections between the terminals and the substrate.
As described in certain preferred embodiments of U.S. Pat. No. 5,518,964, the disclosure of which is hereby incorporated by reference herein, numerous connections can be made between a microelectronic element and a component such as an interposer by providing leads on the connection component so that one end of each lead is permanently attached to the component whereas another end is releasably attached; juxtaposing the component with a microelectronic element such as a chip or wafer and bonding the tip ends of the leads to the contacts on the microelectronic element. The microelectronic element and interposer are then moved away from one another, typically through a predetermined distance, so as to deform the lead towards vertically extensive disposition. In certain other embodiments taught in the ""964 patent, the leads are provided on the microelectronic element rather than on the interposer.
In many of the processes disclosed in the aforesaid patents, an encapsulant is injected around the leads. For example, as taught in certain embodiments of the ""964 patent, such an encapsulant may be injected between the interposer and the microelectronic element during or after the movement step so as to form a compliant layer there between surrounding the leads. As described, for example, in commonly assigned U.S. Pat. Nos. 5,706,174 and 5,659,952, the disclosures of which are also incorporated by referent herein, a compliant layer may be formed by providing a porous resilient layer such as a set of compliant standoffs between the connection component and the microelectronic element and connecting the contact pads of the microelectronic element to leads or other conductive features on the connection component. Typically after these connections have been made, a flowable material such as a curable encapsulant is introduced into the porous layer as, for example, between the compliant standoffs and cured so that the flowable material and the original porous layer form a composite complaint layer. The flowable material desirably also encapsulates the electrical connections.
U.S. Pat. No. 5,798,286, the disclosure of which is also incorporated by reference herein describes, in certain embodiments, techniques wherein a plurality of individual semiconductor chips or other separately formed microelectronic elements are assembled to a single connection component or interposer, as, for example, a dielectric sheet having leads thereon with releasable tip ends as discussed above with respect to the ""964 patent. In certain disclosed embodiments of the ""286 patent, the individual chips are held in a heated chuck and engaged with the tip ends of the leads so that bonding materials such as eutectic bonding alloys, solders or the like carried on the chip or on the leads are activated to form bonds between the chip and the leads. As also disclosed in certain embodiments of the ""286 patent, and in certain embodiments of commonly assigned U.S. Pat. No. 5,766,987, the disclosure of which is hereby incorporated by reference herein, a covering layer may be provided over the rear surfaces of the chips so as to protect the rear surfaces of the chips from encapsulant contamination during the process.
The processes provided by the aforementioned patents provide substantial improvements in the art. Nonetheless, even further improvement would be desirable. For example, the heat applied during a bonding process may cause polymeric layers incorporated in components such as interposers to expand, making it more difficult to achieve precise alignment between the conductive features of the microelectronic elements and the conductive features of the connection component. Certain bonding materials, particularly solders, require fluxes at the connections to make a sound joint. These fluxes can cause problems in service unless they are removed by a cleaning process, which adds cost to the process. Also, many of the techniques commonly used in dispensing solder onto parts to be assembled do not lend themselves to extremely fine-pitch assembly work. Therefore, further improvements in bonding processes and in processes for applying bonding materials such as solders would be desirable.
One aspect of the present invention provides improved processes for making electrically conductive bonds between microelectronic components. In preferred processes according to this aspect of the invention, bonds such as solder joints are formed between contacts on a first microelectronic element such as a semiconductor chip or wafer and conductive elements of a second microelectronic element such as a connection component, by momentarily heating the first microelectronic element so as to activate a bonding material and then allowing the first microelectronic element to cool, leaving the contacts on the first microelectronic element bonded to conductive features on the second element or connection component. In preferred processes according to this aspect of the invention, the second element or connection component is maintained at an average temperature below the average temperature of the first microelectronic element during the momentary heating step. Stated another way, the temporary heating steps is performed so that the bonds are formed while the first microelectronic element is at a higher temperature than the second element or connection component. The microelectronic elements are not in thermal equilibrium with one another. Most preferably, where the second element or connection component includes a body formed wholly or partially from a polymer, the second element is maintained at an average temperature below the glass transition temperature of the polymer. This markedly reduces dimensional changes in the polymeric body.
Most desirably, the temporary heating and cooling steps are performed while the elements are maintained under a vacuum, i.e., a subatmospheric total pressure from most typically about 10-50 milliTorr. The vacuum inhibits heat transfer between the microelectronic elements. The first microelectronic element desirably is heated by directing radiant energy onto it. For example, where the first microelectronic element is a wafer or chip having a back surface facing away from the second microelectronic element or connection component, radiant energy such as infrared radiation in a wavelength band absorbed by the material of the chip or wafer can be directed unto the back surface. This heats the wafer rapidly.
A further aspect of the present invention provides methods of applying bonding material such as solder to form bonding material masses or xe2x80x9cbumpsxe2x80x9d on a microelectronic element such as a chip or a wafer. In preferred methods according this aspect of the invention, the element has a non-wettable surface surrounding the contacts, i.e., a surface which cannot be wetted by the bonding material. The bonding material is applied in liquid form onto the element, such as on the contact bearing front face of the chip or wafer, so that the surface as a whole, including both the contacts and the non-wettable surface, is exposed to the liquid bonding material. The liquid bonding material adheres only to the contacts. The liquid bonding material forms small droplets having the natural shape of a meniscus due to surface tension. As further explained below, these droplets may have a very small volume. For example, the volume of such droplets may be substantially smaller than the volume of a spherical solder ball having a diameter equal to the diameter of the contact. Small solder masses, such as the meniscus-shaped solder masses, greatly reduce the probability of shorting between adjacent contacts during a bonding operation. They are particularly valuable with small contacts as, for example, contacts having diameters less than about 100 microns and/or contacts spaced at center to center distances or xe2x80x9cpitchxe2x80x9d less than about 200 microns.
A further aspect of the present invention provides a method of preparing a microelectronic element including the step of providing a microelectronic element having a plurality of contacts arranged in one or more rows, the contacts in each row being spaced apart from one another in a row direction. The method further includes applying a liquifiable bonding material as, for example, a solder paste to the element in elongated strips extending across the contacts transverse to the row direction. The lengthwise or elongation direction of each bonding material strip is transverse to the row direction. Desirably, the strips have widthwise dimensions in the row direction less than the dimensions of the contacts in the row direction. The method desirably includes the further step of bringing the bonding material to a liquid condition as, for example, by heating the bonding material. When the bonding material is in a liquid condition, it wets the contacts preferentially and does not substantially wet a surface of the microelectronic element surrounding the contact. Thus, surface tension in the liquid bonding material forms the bonding material in each strip into a mass covering the contact crossed by such strip. Applying the bonding material in the form of elongated strips transverse to the row direction provides spaces between the edges of adjacent bonding material strips substantially greater than the spaces between adjacent edges of the contacts in the row. Also, the spaces between edges of adjacent bonding material strips is greater than the spacing which would exist between adjacent edges of bonding material masses if the bonding material were applied in the form of a circular or square blob. This large spacing between bonding material masses prior to liquefaction allows application of the bonding material by techniques having limited precision such as stenciling without causing adjacent boning material masses to touch one another. When the bonding masses are liquefied, they will be effectively confined to the areas encompassed by the individual contacts and will remain out of its contract with one another. This technique allows application of solder pastes and other liquifiable bonding materials on contact space at a fine pitch. It also allows formation of relatively small, meniscus-shaped solder masses as discussed above. In a variant of this approach, the liquifiable bonding material is applied is masses which touch individual contacts in the row but which have their centers of area offset from the row centerline. Alternate masses have centers offset in opposite directions from the row centerline. The masses may not include elongated strips. Here again, there are relatively large spaces between adjacent masses.
A further aspect of the present invention provides methods of making microelectronic assemblies. A method in accordance with this aspect of the invention desirably includes the steps of providing one or more microelectronic elements and one or more components with a deformable barrier such as a flexible sheet. These are provided so that conductive features of the microelectronic elements and components confront one another in a working space at least partially bounded by the deformable barrier. The method according to this aspect of the invention further includes the step of maintaining the partial pressure of oxygen below the partial pressure of oxygen providing in normal atmospheric air at atmospheric pressure, i.e., below about 160 Torr, and maintaining the working space under a total absolute pressure lower than the total absolute pressure prevailing outside of the working space. A pressure differential on the barrier urges the barrier into the working space and the barrier urges conductive features on the one or more microelectronic elements into engagement with conductive features on the one or more components. A bonding material such as solder is activated at the engaged conductive features. This activation occurs at least partially while the atmosphere within the working spaces maintained at the aforesaid low partial pressure of oxygen. Most preferably, the total absolute pressure within the working space is below normal atmospheric pressure, and the activation step is performed while ambient atmospheric conditions prevail outside the working space. The desired low partial pressure of oxygen can be achieved simply by withdrawing air from the working space to lower the total pressure or, more preferably, by first flushing the working space with a non-oxidizing gas and then bringing the working space to the desired subatmospheric total pressure. The step of activating a bonding material desirably includes momentarily heating the bonding material and the conductive features as, for example, by applying radiant energy.
As further discussed below, connection of the bonds under a low oxygen partial pressure facilitates bonding, and particularly solder bonding, without flux. Because the deformable barrier in conjunction with other components, encloses the working space, the process can be performed under normal ambient atmospheric conditions, outside of a vacuum or pressure chamber. For example, where the components and microelectronic elements are heated by application of radiant energy, this step can be performed outside of a vacuum chamber. Moreover, the mechanical action of the deformable barrier or film urging the conductive features together helps to assure reliable engagement and bonding even where the parts carrying the conductive features are out of plane or of an uneven thickness. The deformable barrier may be a film or other deformable element formed separately from the microelectronic elements and components. Alternatively, the component or components may serve as part of all of the deformable barrier. For example, where the component or components includes a flexible, sheet-like dielectric element carrying conductive features, the dielectric element itself may serve as the deformable barrier.
A related aspect of the invention provides additional methods of making microelectronic assemblies. A method according to this aspect of the invention includes the step of providing one or more microelectronic elements in a working space between a flexible film and one or more components so that a front face of each microelectronic element with conductive features exposed thereon confronts a front face of a component having conductive features exposed thereon such front face and so that a rear surface of each microelectronic element faces upwardly away from the one or more components and toward the film. The conductive features of the elements and components are aligned with one another. This aspect of the invention further includes the step of maintaining the working space under an absolute pressure less than the absolute pressure prevailing outside of the working space, so that the film urges the microelectronic elements downwardly against the components and thus biases the conductive features into engagement with one another. While this pressure differential is present radiant energy is directed into the working space, preferably through the film and onto the one or more microelectronic elements to thereby momentarily heat the engaged conductive features and activate a bonding material on the engaged conductive features so as to bond these features to one another. Here again, the step of maintaining the working space desirably includes maintaining the working space at a subatmospheric total pressure, and the step of directing radiant desirably is performed while the exterior of the film is exposed to ambient atmospheric pressure. Here again, because radiant energy exposure operation and hence bonding can be performed outside a vacuum chamber, the process can be performed at low cost, using simple equipment. Moreover, the components and microelectronic elements can be cooled rapidly when exposure to radiant energy is terminated.
Most preferably, the step of providing the microelectronic elements and components includes providing the one or more components on an upper surface of a fixture and the step of maintaining the working space at subatmospheric pressure includes sealing a peripheral region of the film to the one or more components or to the fixture and withdrawing gas from the working space.
The flexible film desirably is sealingly connected to the rear surface or surfaces of the one or more microelectronic elements. The method may further include the step of injecting a curable material between the flexible film and the one or more components so as to encapsulate the conductive features after bonding. Thus, the same flexible film which bounds the working space during the bonding step also serves to protect the rear surfaces of the microelectronic elements during the encapsulant injection step. The conductive features on the microelectronic elements, on the components or both, may include leads, and the method may further include the step of moving the microelectronic elements and components away from one another to thereby deform the leads. This step may occur, for example, concomitantly with encapsulant injection. Particularly preferred methods according to this aspect of the invention include the step of temporarily securing the microelectronic elements to the one or more components prior to activation of the bonding material as, for example, when the microelectronic elements are first placed onto the one or more components. The temporary attachment is detached during or after the bonding step as, for example, before or during the step of moving the components and microelectronic elements away from one another.
A further related aspect of the invention provides methods of making microelectronic assemblies including the step of temporarily securing one or more elements in place on one or more components by providing temporary securements extending between the microelectronic elements and components. The temporary connecting elements desirably adhere to the microelectronic elements and to the components. The method according to this aspect of the invention desirably further includes the step of connecting conductive features of the one or more microelectronic elements to conductive features of the one or more components and releasing the temporary securements during or after the connecting step. Typically, the conductive features are exposed on front faces of the one or more microelectronic elements and on first surfaces of the one or more components and the temporary securing step is performed so that the front faces of the microelectronic element or elements confront the first surfaces of the one or more components, so that the conductive features are aligned with one another and so that the temporary connecting elements extend between the confronting surfaces of the microelectronic elements and components. According to this aspect of the invention, microelectronic elements such as chips can be aligned with the components and bonded to under moderate conditions, desirably at normal ambient room temperature. This simplifies the task of placing the microelectronic elements accurately. Once the microelectronic elements are disposed in the correct positions relative to the components, flexible film as discussed above can be placed over the rear surfaces of the microelectronic elements without moving them, inasmuch the microelectronic elements are held in place by the temporary securement.
In some embodiments the temporary securements can be released by application of heat. For example, where an encapsulant is applied around the conductive features and brought to an elevated temperature, the temperature used to cure the encapsulant may be sufficient to release the temporary securement. Alternatively or additionally, momentary heating applied to bond the conductive features to one another may serve to release the temporary securements from adhesion or to destroy the temporary securements. The temporary securements typically include organic materials such as thermoplastics or other polymers which can be degraded by application of heat.
A further aspect of the invention provides components for use in making microelectronic assemblies, as, for example, in the processes discussed above. A component according to this aspect of the invention includes a dielectric body having conductive features exposed at a first surface and also includes one or more xe2x80x9cdummy padsxe2x80x9d exposed at the front surface. The dummy pads may be formed from conductive or dielectric material. The dummy pads have release portions which are releasably connected to the dielectric body. One or more masses of temporary securement material are provided on the release portions of the dummy pads. The temporary securement material may be an adhesive material or other material adapted to bond to a microelectronic element when the microelectronic element is disposed over the front surface of the dielectric body. As further discussed below, the dielectric body may have holes extending through it to the dummy pads and the release portions of the dummy pads may be disposed in registration with the holes. Anchor portions of the dummy pads extend beyond the holes and onto the dielectric body. In use, the release portions of the dummy pads may break away from the anchor portions of the dummy pads.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.